Down Syndrome Congenital Chromosomal Disorder Biology Essay

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Down syndrome is a congenital disorder that is caused by a superfluous chromosome. Downs syndrome is the most common cause of deformity and mental retardation in an infant (Frey 1206). There are three different types of Down syndrome: Free trisomy 21(or Trisomy 21), Translocation, and Mosaicism. In this paper I will be specifically focusing on Trisomy 21. The fact that Down's syndrome is caused by the existence of another partial or whole extra 21st chromosome is the reason why it can also be called Trisomy 21. As a result of having three 21st chromosomes this would give a patient who has Trisomy 21 forty-seven chromosomes, instead of a normal total of 46 chromosomes. Within the United States there is an estimated 6,000 children born with Down's syndrome every year (Frey 1206). Statistically this breaks down to about 1 out of every 800 infants that are born with Trisomy 21(Driscoll 2556).

Down syndrome occurs during meiosis. During this stage disjunction would normally occur. Disjunction is when a pair of chromosomes are supposed to split up and divide into two different cells; however, in Down syndrome the whole pair of chromosomes goes to one cell and does not divide. As a result, one cell will have 24 chromosomes and the other will have 22 chromosomes. This uneven distribution of chromosomes is called nondisjunction. If one of the gametes has an abnormal number of chromosomes and unites with its counterpart during fertilization, this will result in a zygote with an abnormal number of chromosomes. One of the gametes has two 21st chromosomes and its counterpart has one. When these unite and fertilize, they result in the zygote having three 21st chromosomes. As a woman ages, so does the frequency of nondisjunction (Driscoll 2556).

Even though Down's syndrome is both a congenital and a chromosomal disorder; unlike some diseases which cannot be detected upon physical examination, Down's syndrome is identifiable by many common outward physical characteristics. The clinical manifestations of this disease are various minor and major differences in body configuration such as a noticeable difference in facial features which would include the following characteristics: flat looking face and bridge of the nose, a low set nose that is smaller in size, small head, small mouth that causes the tongue to appear abnormally large and to stick out, almond shaped eyes that slant upward, rounded cheeks, a sloping under chin, epicanthal folds (which are extra folds of skin near the nose region that can be seen at the inside corner of each eye), and low set malformed ears (Frey 1207). More than half of those diagnosed with Down syndrome have hearing problems. Manifestations of disease can also be observed on the hands of a patient with Down's syndrome. The difference in hand features that normally occur are: abnormally small and wide hands, a deformed fifth finger, and a very noticeable simian crease which is a abnormally deep crease that is located across the center of the patients palm. The feet are also usually affected and a large gap is usually observable between the big and second toe, also abnormal creases can be seen on the soles of the patients' feet. Other noticeable signs of Down's syndrome are patients are usually shorter than normal height, have a shorter neck, shorter limbs, poor muscle tone, and are double-jointed (Frey 1207).

There are also many clinical manifestations that cannot be phenotypically expressed. Variations of intelligence levels can be observed in Down's syndrome patients. Lower than average cognitive abilities can be exhibited or minor to severe mental retardation can also occur. Gastrointestinal manifestations of Down syndrome affect about 5-7% of Down Syndrome. Duodenal atresia, which is a obstructed duodenum, is the most common gastrointestinal tract malformation. This often causes the baby to vomit and impairs its ability to gain weight properly due to the fact that the babies food is not leaving the stomach and entering the intestine for digestion properly. Heart defects are also common in people diagnosed with Down syndrome. Atrial septal defects, ventricular septal defects, and Tetralogy of Fallot are some of the heart defects that frequently accompany Down syndrome. Patients with Down syndrome also have an increased chance of developing hearing loss, visual problems, pneumonia, thyroid problems (most commonly hypothyroidism), ear infections, and other various types of infections. Down syndrome patients also have an increased risk of developing leukemia. They are twenty times more likely to develop leukemia (Frey 1207).

Prenatal screening can be done to help diagnose if the fetus has Down syndrome. Both noninvasive screening test and invasive diagnostic testing are available; however, screening tests are less accurate than diagnostic testing. If the expecting mother wants a definitive answer than "she may decide to bypass screening and choose either chorionic-villus sampling at 10-12 weeks gestation or amniocentesis between 15-20 weeks' gestation" (Driscoll 2561). Noninvasive and invasive tests are done so that the parent(s) can either start preparing for a baby with Down syndrome or safely terminate the pregnancy (Frey1209).

One of the noninvasive screening tests that can be done is serum screening tests, which would be performed in the second trimester. This test measures different biochemical markers and is sometimes referred to as quadruple screening. This test "has a detection rate of 80% for Trisomy 21" and is presently "the most common screening test performed between 15 and 22 weeks' gestation" (Driscoll 2557). The quadruple screening test is done to measure the levels of alphafetoprotein, human chorionic gonadotropin (hCG), inhibin A, and unconjugated estriol. In Trisomy 21 characteristically low maternal levels of alpha-fetoprotein and unconjugated estriol can be seen and high levels of inhibin A and hCG are detected.

Chorionic-villus sampling and amniocentesis are two types of invasive diagnostic testing that may be used to help diagnose Down syndrome prenatally (Driscoll 2559). During chorionic-villus sampling a small tube is inserted into the uterus to obtain a small sample of the placenta. Amniocentesis is a procedure where a thin long needle is used to withdraw amniotic fluid from the amniotic sac (Frey 1209). Although these two types of tests are more invasive they are more accurate and have a relatively low rate of pregnancy loss (Driscoll 2559). There is a slightly increased risk of fetal loss with chorionic-villus sampling than with amniocentesis" (Driscoll 1472). Contemporary rates of pregnancy loss after ultrasound-guided amniocentesis are lower than the rate of 1 per 200 pregnancies (0.5%)" and "rates of fetal loss associated with chorionic-villus sampling are about 1% higher than rates associated with amniocentesis" (Driscoll 2559). Although there is more of a risk factor when testing for Down Syndrome by means of Amniocentesis and Chorionic-villus sampling, they are more effective than noninvasive methods because these tests provide definitive answer by means of Karyotyping.

Down syndrome is usually already the suspected diagnosis for those who have undergone prenatal testing. Also at birth when the physical signs of Down syndrome can be seen, the infant then undergoes genetic testing to verify that they have Trisomy 21. Genetic testing can be done via tissue sample or blood sample. The sample of tissue or blood is used to obtain a karyotype, a picture of the chromosomes paired and arranged by side and shape, of the infant's chromosomes. A karyotype done to test for Trisomy 21 would reveal that there is an extra chromosome 21 (Frey 1208).

Down syndrome is a non-curable disease, thus there is no treatment available. However, the secondary complications that patients experience due to Down syndrome are the symptoms that can be treated. For example, hearing impairment is often treated with hearing aids. Visual problems can be corrected with glasses or surgery (Royston 20). Duodenal atresia and heart defects are treated by surgery (Frey 1208).

One way that doctors do try to prevent the occurrence of Down syndrome is that they refer couples preparing to have babies to a genetic counselor (Frey 1209). The counselor informs the couple that the risk of having a baby with Down syndrome increases as the mother's age increases; however, the majority of infants with Trisomy 21 are born to women younger than 35 years of age (Driscoll 1471). Also, prenatal testing may aid in diagnosing whether the fetus has Trisomy 21 or not. This advanced information presents the expecting parents an opportunity to choose what outcome is best for them. The expecting parent(s) may choose to begin planning on how to care for an infant with Down's syndrome or a safe abortion of the fetus is also another option that is made available (Frey 1209).

The prognosis for patients with Down syndrome varies due to the fact that not all Down syndrome patients will be afflicted with the same complications. Some people diagnosed with Down syndrome, depending on the severity of the disease, are able to work, go to school, and live almost completely independently. "Provided they are helped and supported, either by their families or by social services, people with Down syndrome can live the same kind of life as everybody else" (Royston 37). When you look in terms of the reproductive ability of men and women affected with Down syndrome an immense difference can be seen. Men almost always are sterile, which enables them to have children. In contrast, women affected with Down syndrome have the ability to have offspring; however, about 50% of infants born to a mother with Down syndrome will be diagnosed with Down syndrome as well (Frey 1208). Age 50-55 is the average age of death for a person diagnosed with Down syndrome (Frey 1208). 80% of those diagnosed with Down syndrome live to the age of 60 or older (Driscoll 2557).

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